In conclusion, 4 display can be connected to 4s. I am using like this, now.

But you need some trick.

iPhone 4 display assembly can not bind to 4s frame directly because 4's display assembly binding legs location are not matched to the 4s's location. You can tear the white(or black) plastic frame of 4 display assembly. Binding legs are attached to this assembly's plastic frame. If then, you can connect 4 display to the 4s body.